Download Kinetic Energy

Energy
Energy
• “Something” that enables an object to do
work.
• When you have “Energy” you can do work
• The amount of energy that an object posses
is related to the amount of work the object
can do.
• SI unit is Joules (J) – (N*m)
• Comes in various forms like ……..
Five main forms of energy:
• Chemical
• bonds atoms and ions together.
• Electromagnetic - Electrical
• moving electric charges.
• Electromagnetic - Light
• Energy in the form of light
• Mechanical
• associated with motion.
• Heat/Thermal
• internal motion of particles.
• Nuclear
• in the atomic nucleus.
• Nuclear fission & Nuclear fusion
Mechanical Energy
• Energy due to the
position, or
movement, of
something
• Mass is, or will be,
displaced (moved)
• Kinetic or Potential
• Non-Mechanical
• Mass is not displaced,
no movement or
velocity gained or lost
• Examples
• Chemical Energy
• Gasoline
• Food
Potential Energy
• Gravitational
• Energy an object has Potential Energy
due to position
• due to gravity acting
• Stored Energy
on an object,
• Potential Energy
• Stored Work
• Examples
• Why do we not walk
under ladders
• Scared of Rubber
Bands
PE = mgh
PE = mass x gravity x height
Metric unit of energy is
Joules (same as for work).
Sample Problem
6 kg
Gravitational Potential energy is
PE = (mass) x (Gravity) x ( Height )
= (6 kg)x (10m/s/s ) x ( 20 m )
= 1200 Joules
What is potential energy on the ground?
At a h = 0 m, PE is zero Joules.
20 m
What is the potential energy of a 6kg bowling
ball at a height of 20 meters?
Kinetic Energy (KE)
• Energy associated
with an object due to
its motion.
• If something is in
motion it has the
ability to do work
• Examples
• How do you stop
something that is
moving
• A stationary object
has zero kinetic
energy
2
KE = ½ms
KE= ½ x Mass x Speed2
Notice that speed is
squared, If speed is
doubled what happens
to the KE?
Sample Problem
20 m
What is the kinetic energy of a 6kg
bowling ball, falling from a height of 20
meters with a speed of 20 m/s, just as it
reaches the ground?
v= 20m/s
6 kg
Kinetic energy is
(Kinetic Energy) = ½ ( Mass ) x ( Speed ) x (Speed)
= ½ x( 6 kg ) x ( 20 m/s ) x ( 20 m/s )
= 1200 Joules
20 m/s
Conservation of Energy
• Law of Conservation of Energy states that
energy can never be created or destroyed. It
can only be transformed from one form into
another.
• Example
• A book falls from a table
• Draw out and label in Notebook
• The total Mechanical Energy must always be
equal to the sum of Kinetic Energy and
Potential Energy
Etotal = KE + PE
Pendulum
“Like a child on a swing”
Energy exchange from PE to KE and back.
Maximum
Height
Check it out!
Can predict speed from height
since PE+KE constant
Maximum
Speed
Maximum
Height, again
More than Meets the Eye
Watch below as the KE and the
PE change as the car changes
height and speed. Notice that no
matter when you look at the two
bars, there is only the same
amount of energy that you
started with.
Watch This!
Check for Knowledge
• Suppose you have a 1 kg mass at a height of
1.25 m. How much potential energy does it
have?
• The mass is now let go. At the very bottom,
what is it’s kinetic energy?
• What is the speed of the mass just before it hits
the ground?